1. Field of the Invention
The present invention relates to systems and methods for phase noise measurement in transmitted signals, particularly signals using layered modulations.
2. Description of the Related Art
Digital signal communication systems have been used in various fields, including digital TV signal transmission, either terrestrial or satellite. As the various digital signal communication systems and services evolve, there is a burgeoning demand for increased data throughput and added services.
The conventional method of increasing spectral capacity is to move to a higher-order modulation, such as from quadrature phase shift keying (QPSK) to eight phase shift keying (8PSK) or sixteen quadrature amplitude modulation (16QAM). Unfortunately, higher-order modulations typically have a greater sensitivity to the presence of phase noise.
Along the propagation path of a transmitted signal in a wireless system, a low noise block (LNB) provides a link between the wireless and landline environments, among its other functions. The LNB translates the radio frequency (RF) input frequency to an intermediate frequency (IF) output through a low-cost local oscillator. Consequently, the LNB is usually the dominant source of phase noise along the communication signal path. It is important that LNB phase noise not exceed the specification for the allowable total communication link noise to perform at the expected level. However, measurement of LNB phase noise has been an expensive proposition because of the wireless interface. In the implementation of digital broadcast systems, particularly using higher-order modulations, measurement of phase noise in the signal can be important in testing components and system function.
In a conventional method of measuring phase noise, the RF input of the LNB may require an anechoic chamber to perform an adequate phase noise measurement. Furthermore, such a measurement is usually accompanied with a time-consuming testing procedure over a range of operating input frequency of interest. The invented method calculates a phase noise spectrum with a single capture of a short received signal. The software that calculates the LNB phase noise is both simple and efficient.
Accordingly, there is a need for systems and methods that allow an on-line measurement of phase noise, particularly in digital television applications. Further, there is a need for such methods and apparatuses to be convenient and inexpensive to use. As detailed hereafter, the present invention meets these and other needs.